1. Field of the Invention
The present invention relates to the field of electrical resistivity tools, which are used to measure certain properties of earth formations penetrated by boreholes. More specifically, the present invention relates to a system for digitally processing signals in electrical resistivity tools to improve the accuracy of measurements made by the tools.
2. Discussion of the Relevant Art
Electrical resistivity tools are used to make measurements of electrical resistivity of earth formations penetrated by boreholes. Electrical resistivity measurements can be used for, among other things, estimating content of various types of fluids which can be contained in pore spaces in the earth formations.
Electrical resistivity tools known in the art include galvanic devices. Galvanic devices typically comprise electrodes placed on an insulating exterior surface of the tool. All the electrodes on the tool typically make electrical contact with the earth formation through a conductive fluid which fills the borehole. Some of the electrodes are connected to circuits in the tool which generate electrical current. Other electrodes are connected to different circuits in the tool which measure voltage differences and current flow magnitudes. Measurements of voltage difference and current flow can be related to the electrical resistivity of the earth formations.
A galvanic instrument known in the art is called a dual laterolog tool. The dual laterolog tool comprises electrodes which emit measuring current, and focusing electrodes which emit focusing currents used to constrain, or focus, flow of the measuring current in a predetermined pattern. By focusing the measure current in a predetermined pattern, measurements of resistivity can be related more precisely, for example, to thin vertical sections of the earth formation. Other predetermined patterns for focusing the measuring current can enable measurement of formation resistivity at a radial distance closer to the borehole, which can be useful for estimating movement of fluid from the borehole into the pore space in the formation. A description of the typical electrode arrangement and current focusing patterns of the dual laterolog tool can be found for example in "Introduction to Wireline Log Analysis", by Ed L. Bigelow, Atlas Wireline Services, Houston, Tex., 1992 (p. 59).
The tool described in the Bigelow reference, for example, simultaneously makes two differently focused measurements of resistivity using the same set of electrodes. The simultaneous measurements are performed by using measuring and focusing current systems operating at two different frequencies. Each of the measuring and focusing systems operates at a different one of the two frequencies.
In order for the dual laterolog tool to accurately record the measurements made by each focusing and measuring system, the signals generated and detected by each frequency system must, to the greatest extent possible, be prevented from interfering with each other.
Methods are known in the an for reducing interference between measuring and focusing systems operating at different frequencies. One method known in the art is to provide an analog bandpass filter having a very narrow bandwidth to an input of the voltage and current measuring circuits in the tool corresponding to each measurement system frequency. Analog bandpass filters reject passage of electrical current at frequencies other than within a so-called frequency passband. One of the limitations of analog bandpass filters is that they can pass some current at frequencies other than within the frequency passband. A plurality of measurement systems operating at different frequencies may not be sufficiently isolated from each other by using analog bandpass filters to prevent interference between respective systems.
Another method known in the art for reducing interference between different frequency measurement systems is to provide a current source for each measurement system having as nearly as possible only one frequency component. This type of source is called a monochromatic current source. By providing a substantially monochromatic current source for each measurement system, detections of voltages and currents in a particular measurement system which are not at the frequency of that particular measurement system can be reduced.
A system for providing a substantially monochromatic current source is known in the art and is described, for example, in U.S. Pat. No. 4,499,421 issued to Sinclair. The system described in the Sinclair '421 patent comprises a pair of digital latches and a precision resistor network to generate a stair-step approximation of a sinusoidal waveform. The stair-step approximation output from the resistor network is then conducted to an amplifier having an analog low-pass filter. The analog low-pass filter reduces the magnitude of the "stair-steps" since they have a much higher effective frequency than the sinusoidal signal. In addition to the limitations of analog filters as previously described herein, the system disclosed in the Sinclair '421 patent has a further limitation in that the system in the Sinclair patent uses a precision resistor network to accomplish the digital-to-analog conversion. Some of the limitations of precision resistor networks used in digital-to-analog conversion are described, for example, in U.S. Pat. No. 5,357,252 issued to Ledzius, et al. The Ledzius '252 patent states that the "resistive-divider" technique of data conversion, which includes the digital-to-analog conversion of the signal generator disclosed in the Sinclair '421 patent, can be difficult because the resistive-divider technique requires using high precision analog components which may be difficult to form, particularly in a system intended to be used in the limited space provided inside a resistivity tool used in boreholes. A resistivity tool comprising a plurality of different measurement and focusing systems which operate at different frequencies, using a plurality of signal generators similar to the one disclosed in the Sinclair '421 patent, is impractical.
It is known in the art to provide an analog-to-digital converter responsive to a range of frequencies to reduce signal distortion which can be present in analog signal processing circuits. The Ledzius '252 patent, for example, discloses an analog-to-digital converter responsive to a plurality of frequencies defining a usable range called the bandwidth. The analog-to-digital converter in the '252 patent could be used in a multiple frequency resistivity tool if each measuring circuit for each different frequency could be connected to a converter similar to the converter disclosed in the '252 patent and combined with a narrow bandwidth analog filter. However, a limitation on the use of the converter of the '252 patent in a multiple frequency resistivity tool is the need to include analog components in a filter stage of the converter, as shown at 83 and 84 of FIG. 5 in the '252 patent. The converter disclosed in the '252 patent was intended to have a bandwidth comprising a relatively wide range of frequencies in order to be usefull, for example, in digital telephony. Including the analog components of the '252 patent in the output stage of the converter as disclosed in the '252 patent would likely allow interference between the different frequency measurement systems when used in a multiple frequency resistivity tool.
A further limitation on using the converter disclosed in the '252 patent is that the disclosed converter does not eliminate the need for the narrow bandwidth analogbandpass filter provided at the input of the analog-to-digital converter. The limitations of using analog bandpass filters in the measuring circuit of a multiple frequency resistivity tool, as previously discussed, would still apply if the converter in the '252 patent were used in a resistivity tool.
It is an object of the present invention to provide a resistivity measuring tool having a plurality of fully digital measurement circuits, each circuit capable of operating at a different predetermined frequency, in order to provide minimum interference between individual measurement systems.
It is a further object of the present invention to provide a resistivity measuring tool having a plurality of monochromatic current sources each of which is fully digitally synthesized in order to minimize generation of spurious frequencies in the individual measure currents.